This was originally posted on a horrible site called Myspace. When Myspace underwent a redesign in Fall 2010, hundreds of insightful reader comments that had been left over the years were lost. I have since deleted my account there.
Pop quiz: Let's say you have a pile of dry leaves that you raked and want to dispose of. You could burn them in a big fire, or you could compost them and put the compost on your organic vegetable garden. Which method would release more carbon into the atmosphere?
Or, you have a pile of firewood. You could burn the wood in your fireplace, or you could let it decay naturally with the help of fungus and termites. Which choice is more carbon-friendly?
The answer is that in each case, both methods put exactly the same amount of carbon into the atmosphere: all of it. The cellulose and other carbon-containing molecules are broken down and oxidized, whether by fire or by living organisms, and the eventual waste product is carbon dioxide. Burning merely accomplishes this very quickly. Burning leaves and wood also releases other pollutants, which makes burning environmentally less desirable -- but in terms of carbon alone, the carbon in the plant matter will reach the air either way. It's just a question of whether it will take minutes, or months/years.
When people talk about "carbon footprint," they're really referring to how much new carbon a person is responsible for bringing into the environment. Coal, gasoline, and natural gas represent sequestered carbon (in the form of hydrocarbons), having been taken from the atmosphere by plants and animals millions of years ago and stored underground. Burning this material releases carbon that hasn't been in the environment since the age of the dinosaurs, or earlier.
Conversely, with your pile of leaves or firewood, the only way to prevent that carbon from going back into the air would be to bury it far underground, or otherwise sequester it in some way (something that green industries are trying to accomplish with carbon dioxide). If you don't do that, at least in terms of carbon, it doesn't matter how you get rid of it. The carbon is already in the environment, and there it'll stay.